Power circuitry for driving DC (direct current) electric motors is also typically connected to other electronic circuitry including polarity sensitive devices, such as light emitting diodes (LEDs), integrated circuits, electrolytic capacitors and the like.
In simple motorized devices, such as electric fans and shavers, the motor is driven from a source of electrical power. The type of electrical power source selected depends upon the type of motor. The type of motor of concern herein is a DC motor. Such motors typically have a permanent magnet in the stator and brushes to transfer current to the winding on the rotor. A typical power source for such motors may be one or more DC batteries or an alternating current (AC) adapter which converts higher AC voltage to a lower DC voltage utilized by the DC motor.
For economy reasons, ancillary electrical or electronic circuits may also receive electrical power from the same voltage supply as the motor. Such ancillary circuits may include a lamp (incandescent or, more typically, LEDs), a radio, a clock, other motors, a negative ion generator, or the like. Many of these ancillary circuits are polarity sensitive, that is, the positive terminal must be at a higher voltage than the negative terminal to prevent potentially catastrophic damage. For example, LEDs and electrolytic capacitors are two common polarity-sensitive components. Such components tolerate very little reverse polarization without incurring permanent damage.
Problems may occur when these polarity-sensitive components (PSCs) are connected in parallel with the terminals of the DC motor, such as when the current supply is removed from the motor by a mechanical switch or other means of disconnection. The motor windings are inductors and energy is stored in the windings in the form of the associated magnetic fields. Further, the rotor and any flywheel attached to the motor shaft have kinetic energy related to the angular momentum of the rotor and flywheel. When the external power supply is removed from the motor, the motor briefly acts as a generator, which creates a reverse voltage across its electric terminals. Thus, this reverse polarity voltage is then applied to any circuitry, including polarity-sensitive components, connected across the motor terminals.
There are two typical ways to provide reverse-polarity protection in circuits which combine electric motors with ancillary electrical components. The first way is to provide a diode which is ordinarily reverse biased across the motor terminals. This is shown in FIG. 1A. When the power supply is removed, as shown in FIG. 1B, the current flows from the negative motor terminal through the (now forward biased) protection diode back into the motor. Resistance in the motor windings eventually dissipates the energy stored in the magnetic field of the windings and the kinetic energy of the motor rotor along with fan blades, other mechanical loads or a flywheel. A flywheel is a device which may be used for storing energy in the form of angular momentum.
The second method to provide reverse-polarity protection in circuits is to place a non-electrolytic capacitor across the motor terminals, as shown in FIG. 2A. When the external power supply is removed, as shown in FIG. 2B, the current produced by the motor (now acting as a generator with a flywheel) serves to reverse charge the capacitor. The capacitor will, then, discharge through the motor and dissipate energy in the windings of the armature. Of course, the capacitance value must be sufficiently high to absorb the stored energy in the motor, and the kinetic energy of the rotor and flywheel, without raising the voltage across the capacitor and motor to levels which would result in damage to any polarity-sensitive components.
It should be clear then that a reverse polarity condition is always created when the motor current is removed or otherwise interrupted. However, in the case of the protection diode, the voltage may be much lower; such as limited to the forward bias voltage drop of the diode, which is typically 0.7 Volts in the case of a silicon diode. The capacitor could create much higher voltages, especially if its capacitance is low relative to the amount of energy stored in the motor and flywheel. Thus, protection diodes are commonly used to protect ancillary circuitry (or power supply or power control circuitry) from damage from the voltage reversing effects of motors and relays.
This arrangement works well as long as the polarity of the power supply is fixed. However, if the polarity of the power supply is reversed, then all the circuits in FIGS. 1A and 1B will encounter a reverse polarity. In addition to the motor and the polarity-sensitive components, the protection diode may also encounter high reverse voltages. Without any resistance in series with the protection diode, it, too, may fail because of excessive forward current.
One method of protecting the protection diode from excessively high current levels is to ensure that only a proper polarity supply voltage is presented to the motor terminals. This can be achieved by using a full-wave bridge rectifier. Then, irrespective of the polarity of the applied input voltage, the output always has the correct polarization. However, such devices are relatively expensive and increase the amount of energy dissipated even when correct polarity voltage is applied. This is because two diodes in the bridge are always in conduction, thereby causing two voltage drops across the conducting diodes instead of one, as compared to using a single reverse polarity diode.
A general object of the present invention is therefore to provide improved protection for polarity-sensitive components which are connected in parallel with motors or other inductive components.
Another object of the present invention is to provide reverse polarity protection for light emitting diodes, electrolytic capacitors or integrated circuits.
A further object of the present invention is to provide a protection transistor in series with a motor to protect polarity-sensitive components.
Yet another object of the present invention is to provide reverse polarity protection in circuits which include inductors, relays, or the like.
A still further object of the present invention is to provide methods for protecting polarity-sensitive components which are connected in parallel with motors or other inductive components.